Push-pull resistance coupled amplifier



Aug. 4, 1942. D. G. LINDSAY ET AL ,1

PUSH-PULL RESISTANCE COUPLED AMPLIFIER Filed July 13, 1940 2 Sheets-Sheet l All INVENTORS Donald 5. Lindsay BY )2. Fl F1 ATTORNEY.

g- 1942- D. e. LINDSAY ET AL PUSH-PULL RESISTANCE COUPLED AMSLIFIER Filed July 13, 1940 2 Sheets-Sheet 2 mm L:

INVENTOR5 ATTORNEY.

E's tented Aug. 4, 1942 UNITED PUSH-PULL RES Wales STATES PATENT OFFICE ISTANCE COUPLED AMPLIFIER Donald Gordon Lindsay and Reginald Francis Joseph Flood, Sydney, New South Wales, Australia, assignors to Amalgamated Wireless (Australasia) Limited, Sydney, New South Wales, Australia, a company of New South" Application July 13, 1940, Serial No. 345,286

In Australia July 15, 1939 8 Claims. (01. 179-171) plifiers without extremely critical adjustments (which vary with small changes in valve characteristics) of various components therein.

Such diificulties are probably due to the feed back circuits increasing the effects of any small unbalance in the amplifier, or to such circuits feeding back harmonic components normallybalanced in the output circuits. Where feed back circuits are employed in which feed back potentials are taken from the plates of the push-pull valves, the above mentioned unbalance efiects become so bad in many cases as to cause the expected diminution in distortion, due to feed back, to become almost negligible, although the other improvements associated with the use of feed back areususlly obtained.

For a clearerconception of the above mentioned unbalance eiiect attention is now'directed to the following:

In the usual push-pull amplifier where considerable even harmonic potentials are produced in each side of the circuit due to non-linearity of valve characteristics, such harmonic potentials would be completely cancelled if the circuit and output transformer were perfectly balanced.

, Even harmonics due to the cause beiorementioned are in phase at the output plates and the balance cannot be perfect (1. e. some even harmcnics remain) if:

(a) The valve characteristics are dissimilar, resulting in production of diiferent even harmonic amplitudes at the two plates:

(b) The grid drive to the valves is unbalanced;

(c) The output ormer is imperfect and has appreciable loss between one half of the primary andthe other, or is otherwise deficient.

Now consider a practical push-pull amplifier,

wherein there is some appreciable unbalance resulting in even harmonics in the output, and in which negative feedback from one output plate is used. The negative feedback circuit will contain a small amount of even harmonic potentials of the output valves 180 degs. out of phase (1. e. similar to the fundamental signal). As the normal even harmonic potentials produced in the output valves are in phase at the plates it is clear that in one of the output valves only, the even harmonic potentials being applied to the grid must be in such a phase that at the plate of the same valve a cancellation efiect will occur with the harmonic potentials normally present (i. e. without feed back) at this point. This must result in the small unbalance, originally assumed to be present in output plate circuit, being emphasised. This additional unbalance results in more even harmonic potentials being fed back in the degenerati'mnetwprk and so creates a further unbalance and so orir -From this it will be appreciated'that any push-pullcircuit, in which there is initially any appreciable unbalance, can only be further unbalanced by the addition of a single sided degenerative network, the resultant final unbalance being determined by the initial unbalance, the position .at which it occurs, the amount of feed back used,

the gain of the amplifier, and probably various other factors which may appear unimportant in a normal circuit.

A further dimculty in push-pull circuits using feed back from one output valve plate is the production 01 serious hum. Although the hum irom the power supply circuits may not appear in the secondary of the output transformer-due to the balance of the push-pull output circuitthere may be substantial hum voltage on each plate, in the same phase, and this can be conducted through the negative feedback circuits to the input stages so as to reappear in the output as an anti-phase voltage between the .plates. This will then obviously create a disturbance in the output.

One solution to the problem is to use feed back circuits in which the feed back potentials are taken from the secondary side of the output transformer, butsuch an arrangement introduces other difiiculties and has serious limitations in the .field of useful application.

Where feed back potentials are derived, for instance, from the secondary winding of a transformer, the primary of which is connected to the plates of the push-pull valves, unbalance efiects and other deficiencies are largely eliminatedwhen the transformer windings are such that an extremely good balance of coupling is obtained, and they are associated with suitable feed back circuits.

which will be amplified and applied to the grids Where secondary windings, suitable for a range large transformers have to'be used to deal with output power above one watt or so, a well balanced transformer-of suflicient primary inductance is difllcult to build without, at the same time, so reducing the coupling efficiency that instability, due to phase shift, occurs whenever the feed back paths from each output plate to sepa-.

rate electrodes in the valve of a single ended preliminary stage. For example, feed back energy from one output plate is fed to the cathode of the single ended preliminary stage whilst feed back energy is taken from the other output plate to a further electrode in the same single ended preliminary stage.

The phase relationship between the electrodes in the said single ended preliminary stage is such that feed -back currents from the separate paths fiow in opposition through impedances which lie' in the grid circuit of the preliminary valve. Whilst this known arrangement achieves the desired automatic balancing of the output energy, feed back over three stages is not always desirable. Due to the phase rotation in the feed back path the amount of feed back obtainable before instability sets in is not sufiicient for most re'- quirements.

Further, it was not possible in the foregoing prior arrangement to apply tone control circuits to the feedback chain without introducing undesirable values for such components as would be required and unstable effects when the full amount of tone control was used.

When push-pull driver valves are used in a recirc iit'for use in resistance coupled sistance coupled push-pull amplifierand it is desired to apply feed back potentials'from the plates of the output valves to. the input of the driver valves, it is not possible to app y the ,teachings of the prior art for obtaining automatic balancing t Output I It is therefore the principal object of this invention to provide, in resistance coupled amplifiers employing push-pull driver valves, feed back arrangements for achieving automatic balancing of the output.

; It is a further object of this invention to provide feed back circuits for use in push-pull resistance coupled amplifiers employing push-pull driver valves, which circuits will provide automatic balancing arrangements of. such a nature that large amounts of feed back can be employed without any instability, and this is accomplished by 'means of simple circuits which almost com- The above objects are achievdfinhccordance,

with this invention, by circuits so arranged that feed back from the plate circuits of individual 7 of output loads, are desirable, or where fairly output valves in a push-pull amplifier is madescreen of the second, whilst the feed back energy from the plate of the second output valve is fed to the screenof the first driver valve and the cathode of the second. This arrangement provides automatic balancing of the output and permits the use of a tone control network connected across the two feed back paths.

The invention is more fully described in connection with the accompanying drawings, in which:

Fig. l is a circuit arrangement embodying one practical way of carrying out the invention; and

Fig. 2 is a modification thereof.

Fig. 1 shows the circuit diagram of a resistance coupled push-pull amplifier, employing push-pull driving valves, modified in accordancewith this invention to provide inverse feed back from the output valves to the driver valves.

Push-pull resistance coupled amplifiers are well known and a detailed description of their circuit arrangement and operation is considered unnecessary. Only those parts which comprise the present modification and which are essential for a proper understanding of the invention will be described.

Feed back potentials, in accordance with this invention, are taken from the plate of the valve X to the cathode of the valve A and the screen of the valve B; and from the plate of the valve Y to the cathode of the valve B and the screen of the valve A. 3

The feed back paths may be traced as follows: From the plate of the valve X the feed back potentialspass across the blocking condenser 3 and through the resistor 4 to the junction point 5. From the junction 5 they are fed by parallel paths to the cathode of valve A throughthe resistor 6, and to the screen of valve B through the condenser I. The amount of feed back to both electrodes is determined by suitable choice of the values of the cathode resistor 8 for the valve A, the series resistor 8 and the screen resister 9 for valve B, said resistors completing the aforesaid parallel paths and the feed back arrangement across the output valve X. The relation between the cathode resistor 8 of valve A and they screen resistor! to obtain equal feed back to both electrodes is governed by the triode mu of the valve B. If the triode mu of the valve Bis 20, then the resistors 6-8 in theca-thode of valve A and the resistor 9 in the screen of valve 'B should have a corresponding ratio.

A separate and similar feed back path wherein the corresponding components are identified by the same reference numerals butwith the letter a attached in each case is provided from the plate ofvalve Y to the cathode of valve B and screen of valve A.

The values for the various components which form the feed back network from the plate of valve Y are governed by the same considerations as those already set out with respect to the components which constitute the feed back network from the plate of valve X.

From the foregoing description it will be seen that feedback potentials'from the plate circuit of each output valve are effective in both driver valves. The instantaneous phase relationship the second harmonic an between the voltages on the various electrodes of the driver valves and the feed back potentials applied to them from the plates of the output mm is such. thatfor the fundamental an additiw'rdegenerative effect is obtained,.whilst for automatic balancing effect is achieved which tends to compensate any unbalance which might be present in the output circ t if feed back werenot used. 1,.

In practical embodiment of the invention the desired results were obtained by employing the underrnentioned values for the various components employed in the feed back network when valves known by the trade designations 615G and (MG were employed in the output and driver stages respectivelv of the circuitshown in Fig. 1:

Component Value e ssz's'ssssss Tone control may be applied to the circuit arrangement of Fig. 1 by .connecting a frequency dependent network having a fixed or variable attenuation control across the two feed back paths at the junction points 5-5a. A tone control aircuit connected in this manner forms part of the feed back network.

The tone control network may comprise an appropriate fixed or selective combination of frequency dependent components to produce any desired response characteristic in the output circuit.

In the present example an inductance 26 and capacity 21 are arranged to be selectively connected by switch 28 across a variable attenuation control resistor 29 which is connected in series v with a further resistor 30 across the Junction points 5-50 of the feed back paths.

With the condenser 21 in circuit increased'low frequency feed back is provided and the variable resistance 29 controls the amount. Similarly, with the choke 26 in circuit additional high frequency feed back is provided and again the resistance 29' provides suitable variation. Unlike the usual tone control circuits there .is no increase in distortion at the frequencies which are reduced in amplitude, rather, the reverse is the case.

Another method of applying inverse feedback to the fundamental circuit of a resistance coupied push-pull amplifier, employing push-pull driver valves, so as to compensate for distortion and at the same time secure automatic balancing of any unbalance which might exist in the output circuit without feed back, is shown in Fig. 2.

In the circuit of Fig. 2 the feed back is from the plate of the output valve X to the screen of the driver valve 13 and from the plate of theoutput valve Y to the screen of the driver valve A. The cathode resistance Z has a value which is a good deal higher than would be required for grid bias purposes. Unbalance effects are cancelled by virtue of the degenerative effect in this resistor, such degenerative effect only occurring 33, 34 connected across a portion of the supply source.

Another method of obtaining the required biasing potentials for the grid is to tap the grid resistors Il-Sia (which should be of high value compared to the plate resistors of the previous stage) onto a suitable point on the common cathoderesistor Z, the values of the grid leaks 3l-3la and previous plate loads being so proportioned that a very small loss only of the degenerative effect of the high value cathode resistor Z is occasioned.

Alternatively, the junction C could be grounded in the usualmanner and the cathodes returned to a point in the circuit which is negative with respect to earth by the required amount.

The balancing effect obtained from resistance Z can be demonstrated by connecting aby-pass condenser across it.- The efIectis in general an almost unnoticeable change of gain accompanied by a large increase in distortion.

With the circuit arrangement shown inFlg. 2 and employing the well known 6.176 valves as drivers, there appears to be no. particular advantage in using values of 2 much larger than 10,000 ohms, although values lower than 5,000 ohms produced a marked increase in the distortion at any given output.-

Feed back potentials are applied from the plate of the output valve X to the screen of the driver valve B by means of a potentiometer comprising the resistors 35, 36 connected across the output of valve X. Similarly, feed back potentials from the plate of output valve Y are applied to the screen of driver valve A by means of the potentiometer network 350., 35a, across the output of valve Y.

It will be noticed in this circuit that the usual blocking condensers in the feed back pathare omitted. This was found practical since the feed back networks could be proportioned quite readily to give approximately the correct D. C. potential for the screened grids of the associated driver valves and at the same time provide proper feed back values.

The arrangement of the tone control network across the separate feed back paths is similar in all respects to the corresponding arrangement.

, stage tube, and complementary means for deriving a similar potential from the anode in the other output stage tubefor application to said driver stage tubes in an opposite sense.

2. A push-pull amplifier system comprisin gtwo discharge tubes in a driver stage vland two discharge tubes in a driven stage, means; providing tween each cathode of one driver stage tube and a screen grid in the other driver stage tube, and

.a connection of each feed-back circuit to an ap-:

propriate junction between the resistance element and the capacitance "element of said cathode-toscreen grid connections.

3. A push-pull amplifier. having at least two stages, each stage having a pair of electron dis- I charge tubes, each tube of the first stage comprising at least a =cathode,an anode and two grids, each tube of the second stage comprising at least a cathode, an anode and a grid, and two complementary inverse feed-back circuits each characterized in that it couples an anode of a tube in the second stage through a resistance to a cathode of one tube and capacitively to a grid of the other tube in the first stage.

4. A push-pull amplifier having discharge tubes hereinafter designated A and Bin a first stage and C and D inn a second stage, each of said tubes having a cathode, an anode and a plurality of grids; resistance coupling means for causing tube C to be driven by tube A and for causing tube D to be driven by tube B; a feed-back cit cuit coupling the anode of tube C to the cathode of tube A and also to a grid in tube B; and a complementary ieed-back circuit .coupling the anode of tube D to. the cathode of tube 13 and also to a grid in tube-A.

5. A push-pull amplifier according to claim 4' and: including "frequency-dependent netwoifl,

'onein each of said feed-back circuits.

" grids; resistance coupling means for causing tube C to be driven by tube A and for causing tube D to be driven by tube B; a feed-back circuit constituted by a resistive connection from the anode of tube C to a screen grid in tube B, a complementary feed-back circuit constituted by a resistive connection from the anode of tube D to a screen grid in tube A, resistive connections of each of said screen grids to ground, and a cathode-resistor connection to ground, the last said connection being common to the cathodes of tubes A and B.

8. A push-pull amplifier according to claim 7 and including a frequency dependent network in circuit between the screen grids of tubes A and B.

DONALD GORDON LINDSAY. REGINALD FRANCIS JOSEPH FLOOD. 

